The research described in this proposal is designed to improve the performance of local exhaust ventilation (LEV) systems used to control hazardous emissions from industrial processes. Local exhaust ventilation is one of the principal techniques used in industrial hygiene to control the exposure of workers to hazardous airborne contaminants; as such, the proper design of such systems is critical for the protection of the health of many industrial workers. The performance of Local Exhaust Ventilation systems will be improved by developing a system to measure quantitatively the contaminant capture efficiency, and by testing the measurement system in the laboratory and in the field. Such measurements are not made at this time, so that the adequacy of current LEV system design practices is not known. In addition, a theoretical model of capture efficiency based in potential flow theory will be formulated. The model will address the influence of crossdrafts on capture efficiency, and will form the basis of a theoretical treatment which is flexible enough to examine a wide variety of industrial situations. Estimates of the worker's breathing zone concentrations will be enhanced as a result of the research. Quantified capture efficiencies for specific LEV systems and industrial processes, in combination with respirator protection factors, will enable the development of accurate estimates of the cost of a given level of control for the substance and process in question. Optimal protection for the worker, at an economical cost to industry will be much more attainable.